This above type of rear derailleur has hitherto been disclosed in Japanese Patent Laid-Open Gazette No. Sho 59-156,882, which includes a derailleur body comprising a linkage mechanism provided with a base member, a pair of linkage members and a movable member. The base member is supported swingably through a first horizontal shaft to a fixing member fixed to the bicycle frame. A chain guide having a guide sprocket and a tension sprocket is supported swingably to the movable member through a second horizontal shaft, and first and second coiled springs are provided around the first and second horizontal shafts respectively. The first and second coiled springs balance with each other to set the guide sprocket in proper position in the radial direction of each rear chain gear suitable for shifting the chain thereto.
The conventional rear derailleur is provided with a stopper plate having a second stopper engageable with a first stopper provided at the base member. The stopper plate is fitted onto the first horizontal shaft and the first spring is retained at one end to the stopper plate and at its other end to the base member, so that, even before the derailleur body is fixed to the fixing member, the first spring can previously be held to the derailleur body. The stopper plate is provided with a positioning nose which, when the derailleur is mounted on the fixing member, abuts against an abutting projection provided thereon, thereby setting an initial position of the derailleur body, in other words, an angular phase thereof with respect to the axis of a hub shaft at the multistage rear chain gear assembly, and also setting an initial twist angle of the first spring.
The derailleur body is mounted on a fork end fixed to the bicycle frame, or on a bracket member fixed to the fork end. In either case, the fork end and bracket member each have a threaded bore screwable with the first horizontal shaft and the abutting projection to abut against the positioning nose at the stopper plate as described above. The first horizontal shaft screws with the threaded bore, thereby mounting the derailleur body to the fixing member, i.e., the fork end or the bracket member.
Thus, the initial position of the derailleur body and the initial twist angle of the first spring are determined when the derailleur body is mounted on the fixing member. When a driving chain stretched across a front chain gear and the rear chain gear engages with each sprocket at the chain guide, the first and second spring deflect to balance with each other, therby setting the guide sprocket in its position with respect to the rear chain gear.
The rear derailleur is sold separately as a bicycle part and incorporated in bicycles of various kinds. Although the multistage rear chain gear assemblies are similar in gear constitution, the fork ends are different in shape and size. If the size and shape of the fork ends are not compatable with the derailleur, the derailleur will not be properly mounted with respect to the axis of hub shaft, and in turn the axis of the multistage rear chain gear assembly. Hence, when the chain engages with the chain guide, a twist angle of the first spring will vary, with the result that the first and second springs will be unbalanced with each other and the guide sprocket will shift from its proper position with respect to the rear chain gear, thereby creating the problem in that the speed change efficiency is deteriorated.
The variation in the twist angle of the first spring is caused by the presence of a different distance between the axis of the hub shaft and the abutting projection at the fork end or a bracket member fixed thereto and also by a different tilt angle at the abutting projection, due to the various kinds of fork ends.
In order to solve the problem of variation in the twist angle of the first spring, the first horizontal shaft need only be loosened to correct a mounting angle of the derailleur with respect to the axis of the hub shaft. In this case, however, the first horizontal shaft tightly screwed must be once loosened to correct the angle of the derailleur body with respect to the axis of the hub shaft and thereafter the first horizontal shaft must be tightly rescrewed, thereby creating problems in that the correction is difficult work efficiency lowers, and much labor is required. In addition, the correction causes the positioning nose to disengage from the abutting projection, thereby shifting the derailleur, when in use, from the proper position with respect to the fixing member.
Also, the derailleur may be mounted on the fixing member in the condition in which the positioning nose at the stopper plate does not reliably abut against the abutting projection, thereby creating a problem in that the twist angle of the first spring varies to cause an imbalance between the first and second springs.